1. Field of the Invention
The present disclosure relates to imagers and more particularly to a method for capturing video images and a video imager enabling the global motion of the images to be determined.
2. Description of the Related Art
The spurious motions that accompany the recording of a video sequence considerably affect the quality of the sequence when the latter is then displayed on a screen. Today, numerous portable electronic devices, particularly cellular telephones, digital cameras, PDAs, etc. integrate means for capturing video sequences. Now, such portable devices are subject to trembling and other undesirable motions and the quality of the video sequences they produce is generally mediocre due to an obvious lack of stability of the image.
There are various known techniques for stabilizing the images of a video sequence, among which so-called pre-processing and post-processing techniques are distinguished.
Pre-processing stabilization techniques rely on the use of object lenses having inertially stabilized lenses, which automatically turn so that the incident light flux reaches the imager with an angle of incidence that varies sufficiently slowly to smooth the rapid variations caused by the spurious motions. It is therefore a sort of low-pass tremble filtering, produced optically and occurring at the moment the images are captured.
These techniques procure excellent results but the high cost of the stabilized lenses means they are reserved for top-of-the-range cameras. They are not therefore readily applicable to video devices integrated into low-cost products, such as those mentioned above. Web cameras are also concerned, as these do not generally have this type of lens.
Post-processing stabilization techniques are implemented by software and occur after the stage of capturing the images. They involve implementing image processing algorithms which are executed by integrated computation means such as DSPs (Digital Signal Processors). Post-processing stabilization techniques are generally based on detecting the global motion of the images in real-time, which enables the spurious motions (trembling) to be distinguished from the intentional motion, so as to keep the latter in the video sequence. Intentional motion, such as a zoom effect (narrowing or enlarging the shooting field) or a displacement of the shooting field for example, is generally a slow and steady motion compared to the spurious motion. It can thus be separated from the spurious motion by a temporal filtering operation of low-pass type.
However, to be truly efficient, known post-processing image stabilization techniques require substantial computing power and thus costly computation means such as the above-mentioned DSPs. As a result, although less costly to implement than the optic stabilization techniques, they are rarely implemented in low-cost applications such as video recording in cellular telephones or in web cameras, or even in certain camcorders. These devices therefore enable video sequences of mediocre quality to be produced.